Field
The present invention relates to microelectromechanical devices and especially to a gyroscope structure and a gyroscope, as defined in the preambles of the independent claims.
Description of the Related Art
Micro-Electro-Mechanical Systems, or MEMS can be defined as miniaturized mechanical and electro-mechanical systems where at least some elements have a mechanical functionality. Since MEMS devices are created with the same tools used to create integrated circuits, micromachines and microelectronics can be fabricated on the same piece of silicon to enable machines with intelligence.
MEMS structures can be applied to quickly and accurately detect very small changes in physical properties. For example, a microelectromechanical gyroscope can be applied to quickly and accurately detect very small angular displacements. Motion has six degrees of freedom: translations in three orthogonal directions and rotations around three orthogonal axes. The latter three may be measured by an angular rate sensor, also known as a gyroscope. MEMS gyroscopes use the Coriolis Effect to measure the angular rate. When a mass is moving in one direction and rotational angular velocity is applied, the mass experiences a force in orthogonal direction as a result of the Coriolis force. The resulting physical displacement caused by the Coriolis force may then be read from, for example, a capacitively, piezoelectrically or piezoresistively sensing structure.
In MEMS gyros the primary motion is typically not continuous rotation as in conventional ones due to lack of adequate bearings. Instead, mechanical oscillation may be used as the primary motion. When an oscillating gyroscope is subjected to an angular motion orthogonal to the direction of the primary motion, an undulating Coriolis force results. This creates a secondary oscillation orthogonal to the primary motion and to the axis of the angular motion, and at the frequency of the primary oscillation. The amplitude of this coupled oscillation can be used as the measure of the angular rate.
Gyroscopes are very complex inertial MEMS sensors. The basic challenge in gyroscope designs is that the Coriolis force is very small and therefore the generated signals tend to be minuscule compared to other electrical signals present in the gyroscope. Spurious resonances and susceptibility to vibration plague many MEMS gyro designs.
One challenge in gyroscope design is quadrature error motion. In an ideal gyroscope structure, the primary oscillation and the secondary oscillation are exactly orthogonal. However, in practical devices imperfections occur, causing direct coupling of the primary mode displacement of the seismic mass to the secondary mode of the gyroscope. This direct coupling is called the quadrature error. The phase difference between the angular motion signal and the quadrature signal is 90 degrees, which means that basically the quadrature error could be eliminated with phase sensitive demodulation. However, the quadrature signal can be very large in comparison with the angular motion signal, and may therefore cause unreasonable dynamic range requirements for the readout electronics or phase accuracy of the phase demodulation.
One known method to deal with this error source is electrostatic quadrature cancellation that removes the error signal at the sensor structure, before the quadrature signal is generated. For this, an electrostatic force, exactly in-phase with the primary oscillation and parallel to the secondary oscillation may be applied to the seismic mass. FIG. 1 illustrates a prior art configuration for electrostatic quadrature suppression, introduced in the U.S. Pat. No. 5,992,233. FIG. 1 shows a seismic proof mass with two fingers projecting from opposite sides of the seismic proof mass. Each of the projecting fingers are surrounded by right and left fingers of a stationary electrode, and a small voltage differential is applied between the right finger and left finger of each pair of fingers. An opposite voltage potential may be applied between a right finger and a corresponding left finger such that this voltage difference creates a balancing force to counteract the quadrature error.
This principle is not, however, applicable for gyroscope structures where the primary mode oscillation of a seismic mass is in out-of-plane direction. MEMS devices are typically layered structures where a gyroscope structure is suspended to an underlying or covering body element. It is easily understood that fabrication of a prior art surrounding stationary electrode configuration to provide the necessary compensating voltage differential in a temperature robust manner would be challenging, or even impossible.